In periodic table from left to right (group 1 to group 18 ) acidic strength increases. Al belongs to group 13,Si belongs to group 14,P belongs to group 15 and S belongs to group 16 . So, Al2O3 will be least acidic and So2 will most acidic among then. So, right order will be Al2O3<SiO2<P2O3<SO2